Water electrolysis device

ABSTRACT

Electrolysis device intended to produce hydrogen by the reduction of water, comprising a cathode compartment, an anode compartment, and an element connecting said compartments and allowing ions to migrate between them, 
     the device being characterized in that the cathode compartment contains at least one weak acid capable of catalyzing the reduction and an electrolytic solution, the pH of which is in the range between 3 and 9.

FIELD OF THE INVENTION

This invention pertains to the field of water electrolysis and more particularly, to a water electrolysis device for producing hydrogen.

BACKGROUND OF THE INVENTION

In the field of energy production, taking into account the increase in needs, costs, supply security and environmental risks, calls for more extensive research work on the diversification and optimal use of primary resources (whether they be fossil, nuclear, renewable, etc.). In this regard, hydrogen, which allows energy to be stored and distributed in a convenient manner while causing little pollution, is a good candidate.

For the extensive use of hydrogen as a source of thermal and electrical energy to be economically and ecologically viable, each of the industrial processes involved, from its production to its ultimate use, including its storage and distribution, must nevertheless be developed.

Since hydrogen is not directly available in the environment, it is particularly important to optimally fulfil these criteria during its production, which must be kept competitive (by maintaining relatively low production costs), clean (the process should be non-polluting so as to preserve one of the major advantages of hydrogen), and of optimal energy efficiency (energy consumption should be limited).

One of the techniques for producing hydrogen is water electrolysis, which is generally achieved using one of the following two devices:

-   -   a device for electrolyzing water in an alkaline medium         (essentially, potassium hydroxide at concentrations from 25% to         40% by weight). Although it benefits from considerable         experience, its improvement requires the development of new         materials that fulfil several criteria, including resistance to         corrosion in an alkaline medium, and the ability to catalyse the         reactions taking place on the electrodes in order to obtain a         high current density and a small overpotential. In particular,         achieving a smaller cathode overpotential leads to cathode         activation by forming a catalytically active surface deposit,         typically by depositing nickel onto an iron base. As far as the         anode is concerned, the base, which should be of a more noble         material (nickel steel or bulk nickel), is often coated with a         catalyst, the deposition and stability of which are delicate         issues, which are the object of intensive research. As may be         noted, one drawback of water electrolysis in an alkaline medium         is the need to use a corrosive electrolytic solution, and         electrodes made with costly materials which degrade with time.     -   a device for electrolyzing water in an acidic medium (typically         sulfuric acid). This comprises lead as the conducting material         for the electrodes and manifolds, or noble metal-based catalysts         (such as platinum black) for the cathode and the anode, as well         as a Nafion (perfluoropolymer of sulfonic acid) cation-exchange         membrane. The problems of electrode corrosion caused by the         strong acidity of the medium (typically, negative pH values), of         environmental non-compliance due to the use of lead, and finally         of the high cost of catalysts, have long restricted the use of         acidic medium electrolysis to the production of small quantities         of high purity laboratory grade hydrogen.

SUMMARY OF THE INVENTION

It is accordingly an object of this invention to remedy the problems and shortcomings of the prior art techniques by providing a water electrolysis device which fully satisfies the above mentioned technical, economical and environmental requirements for producing hydrogen.

A further object of this invention is to provide a water electrolysis device that comprises neither a corrosive electrolytic solution nor electrodes made of costly materials which degrade with time.

The object of this invention is to provide an electrolysis device intended to produce hydrogen by the reduction of water, comprising a cathode compartment, an anode compartment, and an element connecting said compartments and allowing ions to migrate between them, the device being characterized in that the cathode compartment contains at least one weak acid capable of catalyzing the reduction, and an electrolytic solution the pH of which lies in the range between 3 and 9.

Advantageously, said pH lies in the range between 4 and 9; preferably, it lies between 6 and 9, and more preferably, it is equal to 8.

The element connecting the compartments may be an electrochemical bridge known in the art, such as a cation-exchange membrane, a ceramic, and the like.

The electrolysis device of the present invention may preferably be proposed in the form of two embodiments which differ in the acid-base conditions of their cathode compartment, namely:

-   -   an electrolysis device wherein the pH of the electrolytic         solution contained in the anode compartment and the pH of the         electrolytic solution contained in the cathode compartment lie         in the range from 3 to 9. The compositions of the two         electrolytes are typically the same. Preferably, the pH of the         electrolytic solution contained in the anode compartment is         substantially the same as that of the electrolytic solution         contained in the cathode compartment, that is, it is in the same         range or has the same value as the pH of the electrolytic         solution contained in the cathode compartment. Such a device is         typically used in the potentiostatic mode.     -   an electrolysis device wherein the pH of the electrolytic         solution contained in the anode compartment is basic and is         preferably approximately 15. This embodiment may optionally have         some of the features of an existing device for electrolyzing         water in an alkaline medium. Further, the pH of the electrolytic         solution contained in the cathode compartment is preferably         approximately 4. Such devices are typically used in the         galvanostatic mode.

Generally, when implementing the invention, the weak acid intended to catalyze the reduction of water may be in the form of a salt (partially or totally dissolved in the electrolytic solution) and/or adsorbed onto the cathode. Of course, according to the pKa of the weak acid and the pH conditions of the electrolytic solution, the weak acid may be partially dissociated between its acid form and its conjugate base, and each of these two species may possibly contribute to the catalytic action.

However, advantageously, the weak acid is selected so that its pKa is at least greater by one unit than the pH of the electrolytic solution contained in the cathode compartment. Under such conditions, it will undergo little or no dissociation. Therefore, all or most of the weak acid molecules preserve their acidic labile hydrogen atom. Since it is this atom which allows the reduction of water to be catalyzed, the catalytic potential of the weak acid is thus optimized.

Moreover, the weak acid preferably has a pKa in the range between 3 and 9, and more preferably, between 3 and 5. Consequently, the hydrogen atom responsible for the catalytic effect of the weak acid is strongly labile and shows an increased acidic character, thus allowing it to better catalyze the reduction of water, which consequently requires less energy to occur.

The above two embodiments may advantageously be combined. For example, glycolic acid, which has a pKa of 3.83 and a high solubility of 11.6 M, may be added to the electrolytic solution in the cathode compartment, which has a pH of 3.

During water electrolysis, OH and H⁺ ions are produced, respectively, in the electrolytic solution contained in the cathode compartment and in that contained in the anode compartment. Preferably, in order for the water reduction to take place with optimal energy efficiency, it is appropriate to prevent or restrict the resulting pH variation. For that purpose, at least one additional weak acid is added as a buffer to the electrolytic solution contained in the cathode and/or anode compartment so as to prevent or restrict pH variation of this solution or of these solutions during the reduction of water. This additional acid, selected as a function of the pH in the compartment to which it is added, may furthermore function as a catalyst for the reduction of water.

Advantageously, because of this additional weak acid, the pH variation of the electrolytic solution contained in the anode and/or cathode compartment does not vary during the reduction of water by more than two pH units, preferably by one pH unit.

Preferably, said additional weak acid has the same chemical structure as the weak acid intended to catalyze the reduction.

Additional objects, features and advantages of the invention will become apparent from the following description, which is given by way of illustration only.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the three accompanying drawings, which are explained in examples 1 and 2 below.

FIG. 1 shows the variation of current as a function of time during constant-potential electrolysis.

FIG. 2 shows the variation as a function of time of the volume of hydrogen produced by electrolyzing an electrolytic solution of “KCl+dihydrogen phosphate”.

FIG. 3 shows the variation as a function of time of the potential across an electrolysis device according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

The following examples were conducted using dihydrogen phosphate in solution as the catalyst for water reduction.

The weak acid may be mineral (such as orthophosphoric acid, dihydrogen phosphate, monohydrogen phosphate, and the like) or organic (such as lactic acid, gluconic acid, acetic acid, monochloroacetic acid, ascorbic acid, hydrogen sulfate, glycolic acid, amino acids, preferably leucine or lysine).

1) ELECTROLYSIS “UNDER NEAR-NEUTRAL CONDITIONS” 1.1 Operating Procedure

An electrolysis device according to the invention has the following features:

-   -   two compartments (anode and cathode compartments) each with a         volume of 125 cm³, containing the same electrolytic solution,         made of Plexiglas, and separated by a Nafion 1135 membrane after         prior cleaning by immersion into boiling distilled water;     -   a working electrode: 316 L stainless steal cathode with a         geometrical surface of 20 cm²;     -   an auxiliary electrode: anode made of a platinum grid, with a         geometrical surface of 20 cm²;     -   a reference electrode: Saturated Calomel Electrode (SCE).

Using said device, electrolysis of water at a constant potential of −1.1 V/SCE and at a temperature in the range from 20 to 25° C. was carried out for 100 minutes on two distinct electrolytes, namely:

-   -   1) a reference electrolytic solution comprising a solution of         KCl (100 mM) at pH=8.0;     -   2) an electrolytic solution according to the invention         comprising KCl (100 mM)+KH₂PO₄ (500 mM) at pH=8.0 (pH adjustment         by addition of KOH).

To recover the hydrogen thus formed, the cathode compartment was sealed by a plug provided with a Teflon gasket and traversed by a pipe opening into a graduated test tube filled with water and turned upside down in a vessel which also contained water. It should be noted that the device according to the invention might also be used for producing oxygen, which would be generated within the anode compartment also sealed in a similar fashion.

FIG. 1 shows the variation of current as a function of time during constant-potential electrolysis of a reference electrolytic solution (denoted “KCl alone”) and of an electrolytic solution having a pH of 8.0 and containing dihydrogen phosphate (denoted “KCl+dihydrogen phosphate”).

The results for the two electrolyses are summarized in Table 1. These data illustrate:

-   -   the small cathode current and the reduced or even non-existent         reduction of the water for the reference electrolytic solution         (1), since no hydrogen evolution was observed. This is to be         compared with the significant cathode current obtained with         electrolytic solution (2), which in this case is reflected by a         noticeable evolution of hydrogen gas;     -   the stability of the cathode current when only the electrolytic         solution (2) is used.

TABLE 1 Electrolyte (1) KCl (100 mM) (2) KCl (100 mM) + KH₂PO₄ (500 mM) Cathode current From 4 to 13 A · m⁻² 1.7 A · m⁻² (very stable) Hydrogen volume No 10 mL produced in observable 70 minutes production Hydrogen 0 mL/hr From 9 to 10 production mL/hr, or 4.5 to 5 rate L/hr/m²

1.2 Computation of Efficiency

During the production of hydrogen from the electrolyte (2), the Faraday efficiency was computed from the data summarized in Table 2. The “raw” Faraday efficiency obtained under these conditions was nearly 72%. Since no production was detected when the experiment was carried out using the reference electrolytic solution (1), the current thus obtained was considered to be a residual current, probably caused by the reduction of the electrode's surface oxides. The decrease in current from 4 to 1.7 A/m² in 70 minutes supported this hypothesis. Therefore, this portion of the current was not used to transform a species in solution, but rather to induce a change in the surface condition of the electrode. For a long-duration process, this portion of the current may be expected to tend to zero when all oxides are reduced (after a few tens of hours). This quantity of electricity was therefore subtracted in order to derive the “corrected” Faraday efficiency that would be obtained after a few hours of electrolysis. By subtracting the residual quantity of electricity, the “corrected” Faraday efficiency was 92%, that is, 92% of the additional electricity consumption induced by the presence of dihydrogen phosphate was used for the production of hydrogen.

TABLE 2 Universal Gas constant R (J/K/mole) 8.314 Faraday constant (C/mole) 96500 Temperature (K) 298 Pressure (Pa) 1.013 · 10⁵  Electrode surface area (cm²) 20 Number of electrons involved per 2 molecule of H₂ produced Electrolysis time considered (sec) 4186 Volume of hydrogen produced (mL) 10 Number of moles of H₂ produced  4.09 · 10⁻⁴ Total quantity of electricity (C/cm²) 5.516 “Raw” Faraday efficiency 71.6% Residual quantity of electricity 1.247 (C/cm²) (KCl alone) Quantity of additional electricity 4.269 induced by the presence of dihydrogen phosphate (C/cm²) “Corrected” Faraday efficiency   92%

The presence of dihydrogen phosphate in solution at near-neutral pH (pH=8.0) enables electrochemical production of hydrogen (4 to 5 L/hr/m²) on stainless steel in the range of potentials for which no production would be obtained without dihydrogen phosphate. More than 92% of the quantity of electricity consumed in the presence of dihydrogen phosphate ions is used for producing hydrogen, which is excellent in terms of efficiency.

Various observations have demonstrated that the weak acid of this invention indeed catalyzed the reduction of water.

For example, at pH=8.0, no pH variation occurred in the cathode compartment during the electrolysis of water although OH⁻ ions were produced. This is because at pH=8.0, dihydrogen phosphate and monohydrogen phosphate were the dominant phosphate species (14% and 86% of this species, respectively) and acted as a buffer (the H₂PO₄ ⁻/HPO₄ ²⁻ couple had a pKa of 7.20). The pH thus being constant, the free proton concentration at pH=8.0 was however consistently very small, at 10⁻⁸ M. Therefore, this concentration could not be responsible for the high cathode current of 13 A.m⁻², which furthermore was much greater than the cathode current of the reference electrolyte (1) (KCl 100 mM), also at pH=8.0.

2) ELECTROLYSIS UNDER “BASIC CONDITIONS” 2.1 Operating Procedure

The following examples were carried out with the same electrolysis device and according to the same operating protocol as described in the preceding example, except that the electrolyses were now conducted at a constant current of −13.5 A/m⁻² on three different electrolytes whose characteristics are summarized in Table 3.

TABLE 3 Electrolyte in the Electrolyte in the Electrolysis anode compartment cathode compartment (I) KOH KOH 25% by weight KOH 25% by weight, pH (reference) 15.0 (II) KOH-PO4 KOH 25% by weight 0.5M KH₂PO₄, pH 8.0 (0.5M) (III) KOH-PO4 KOH 25% by weight 1M KH₂PO₄, pH 4.0 (1M)

2.2 Demonstration of the Electrolysis Device's Stability and Computation of Efficiency

The electrolyses lasted 2 hours, with the temperature ranging from 20° C. to 25° C. in the three experiments. The production of hydrogen, measured as described above, was on average of the order of 10 mL/hr, which corresponds to a “raw” Faraday efficiency of approximately 80%.

The change in potential across the electrolysis device (denoted Ecell) is shown in FIG. 3, which illustrates the change as a function of time of the potential across an electrolysis device in the course of an electrolysis carried out with a constant current of −13.5 A.m⁻², of a reference electrolytic solution (denoted “KOH”), of an electrolytic solution containing dihydrogen phosphate at pH=8.0 (denoted “KOH—PO4 (0.5M)”), and of an electrolytic solution containing dihydrogen phosphate at pH=4.0 (denoted “KOH—PO4 (1M)”). As illustrated in this figure, the presence of dihydrogen phosphate as a catalyst here again allowed the energy efficiency to be improved, since an increase in potential of 200 and 600 mV relative to the reference electrolytic solution (I) was observed in the presence of 0.5 M and 1 M of dihydrogen phosphate, respectively.

Moreover, the potential Ecell remained substantially constant while the production of hydrogen obeyed a linear law as a function of time. This demonstrates the stability of the stainless steel electrode, which showed no change in its surface condition (pollution, adsorption, corrosion, etc.).

The energy consumption during the production of hydrogen from the three electrolytes was computed (Table 4), taking into account the fact that when the energy consumption is expressed in kWh/Nm³, 1 Nm³ corresponds to 1 m³ of gas measured at 0° C. and at atmospheric pressure.

TABLE 4 Average Energy Energy Energy Energy Ecell spent in spent in consumption consumption Electrolyte (V) 2 hours (kJ) 2 hours (kWh) kWh/m³ of H₂ kWh/Nm³ of H₂ (I) KOH 1.90 0.369 1.02E−04 4.6 4.9 (II) KOH—PO4 1.67 0.324 9.01E−05 4.0 4.3 (0.5M) (III) KOH—PO4 1.30 0.252 7.01E−05 3.1 3.3 (1M)

The presence of dihydrogen phosphate in the electrolytic solution contained in the cathode compartment provides an energy gain of 13% and 33% for a concentration of 0.5 M and 1 M of dihydrogen phosphate, respectively.

It should be noted that the energy efficiency is roughly proportional to the weak acid concentration. Therefore, this concentration may advantageously be increased as long as the energy efficiency increases, in particular up to the point where the weak acid precipitates and/or becomes excessively adsorbed onto the cathode.

3) CONCLUSIONS

As illustrated by the above examples, the electrolysis device according to the invention in both of its main embodiments, advantageously leads to excellent Faraday efficiency during the production of hydrogen.

Furthermore, the stainless steel cathodes of the electrolysis device according to the invention do not suffer any observable degradation. The use of an electrolytic solution of moderate pH in the cathode compartment, combined with the catalyzing power of the weak acid it contains therefore permits the manufacture of a high performance electrolysis device which comprises at least one element in contact with the electrolytic solution in the cathode compartment, this element being partially or entirely made of at least one less noble material. A less noble material appropriate in the implementation of the present invention may be selected from the group consisting of the conductive polymers, the oxidized or non-oxidized forms of Fe, Cr, Ni or Co. This material may be included in the composition of parts of the electrolysis device such as electrodes, compartment walls, circuits for circulating the solutions, etc. The element may thus be a stainless steel cathode, preferably made of 316 L stainless steel.

The use, within the scope of the present invention, of at least one less noble material offers the advantages of substantially reducing the manufacturing costs since this type of material is generally less costly than those conventionally used, such as platinum, of optimally satisfying environmental requirements, of increasing the lives of such devices, while achieving excellent hydrogen production efficiency through the electrolysis of water. 

1-14. (canceled)
 15. An electrolysis device intended to produce hydrogen by the reduction of water, comprising a cathode compartment, an anode compartment and an element connecting said compartments and allowing ions to migrate between them, said device being characterized in that said cathode compartment contains at least one weak acid and an electrolytic solution, the pH of which is in the range between 3 and 9, and in that it comprises an electrode in contact with said electrolytic solution in the cathode compartment, said electrode being partly or entirely made of at least one material selected from the group consisting of conductive polymers, oxidized or non-oxidized forms of Fe, Cr, Ni or Co.
 16. An electrolysis device according to claim 15, characterized in that said electrode is a stainless steel cathode.
 17. An electrolysis device according to claim 16, characterized in that said stainless steel is 316 L stainless steel.
 18. An electrolysis device according to claim 15, characterized in that the pH of the electrolytic solution contained in said cathode compartment is in the range between 4 and
 9. 19. An electrolysis device according to claim 18, characterized in that the pH of the electrolytic solution contained in said cathode compartment is in the range between 6 and
 9. 20. An electrolysis device according to claim 19, characterized in that the pH of the electrolytic solution contained in said cathode compartment is equal to
 8. 21. An electrolysis device according to claim 15, characterized in that the pH of the electrolytic solution contained in said anode compartment is substantially the same as that of the electrolytic solution contained in said cathode compartment.
 22. An electrolysis device according to claims 15, characterized in that the pH of the electrolytic solution contained in said anode compartment is basic.
 23. An electrolysis device according to claim 22, characterized in that the pH of the electrolytic solution contained in said anode compartment is approximately
 15. 24. An electrolysis device according to claim 22, characterized in that the pH of the electrolytic solution contained in said cathode compartment is approximately
 4. 25. An electrolysis device according to claim 15, characterized in that said weak acid has a pKa in the range between 3 and
 9. 26. An electrolysis device according to claim 25, characterized in that said pKa is in the range between 3 and
 5. 27. An electrolysis device according to claim 15, characterized in that said weak acid is selected so that its pKa is greater by at least one unit than the pH of the electrolytic solution contained in said cathode compartment.
 28. An electrolysis device according to claim 15, characterized in that said weak acid is selected from the group consisting of orthophosphoric acid, dihydrogen phosphate, monohydrogen phosphate, lactic acid, gluconic acid, acetic acid, monochloroacetic acid, ascorbic acid, hydrogen sulfate, glycolic acid, and amino acids.
 29. An electrolysis device according to claim 28, characterized in that said amino acid is leucine or lysine.
 30. An electrolysis device according to claim 15, characterized in that at least one additional weak acid is added to the electrolytic solution contained in said cathode and/or anode compartment to prevent or restrict the pH variation of said solution or solutions during the reduction of water.
 31. An electrolysis device according to claim 30, characterized in that said additional weak acid has the same chemical structure as the weak acid initially contained in said cathode compartment.
 32. An electrolysis device according to claim 15, characterized in that said cathode compartment is sealed. 